Video Transcript
A galvanic cell consists of a Mg electrode in one-molar Mg(NO3)2 solution and a Ag
electrode in one-molar AgNO3 solution. What is the standard cell potential?
Whenever we want to find the standard cell potential for a galvanic cell, we need to
examine the two half reactions to determine which one is oxidation and which one is
reduction. The ability or potential of a substance to be reduced is called the reduction
potential.
The reduction potentials of many substances and elements have been determined under
standard conditions and relative to the standard hydrogen electrode, whose reduction
potential under standard conditions is made to be 0.000 volts. Standard conditions are one-atmosphere pressure, 25 degrees Celsius, and one-molar
electrolyte concentrations. This gives us standard electrode reduction potential values, 𝐸, with a superscript ⦵
sign indicating standard conditions. These values are measured in volts.
A special table called the electrochemical series lists many of these standard
reduction potentials. Some substances have negative values relative to the standard hydrogen electrode,
like magnesium, and some have positive values, like silver. The relative sizes of two standard reduction potentials tells us which substance has
the greater potential to be reduced. In other words, these values indicate which substance will be more likely to be
reduced in a galvanic cell.
The half reaction with the lower, or more negative, standard reduction potential will
be the oxidation half reaction. So we can see that magnesium will undergo oxidation. So we can flip this arrow left to right to correctly write this half reaction as an
oxidation process, where solid metal loses electrons to form ions. Or we can just flip the entire equation so it reads easily from left to right,
showing the oxidation process.
And the half reaction with the higher, or more positive, standard reduction potential
will be the reduction half reaction. We can see that silver has a more positive value. And so silver ions will undergo reduction and gain electrons to form silver
metal. Silver has a greater potential to be reduced than does magnesium. In this case, the direction of the arrow on the half reaction is correct. This half reaction is already written as a reduction.
The oxidation half reaction occurs at the anode and the reduction half reaction at
the cathode. Now that we know the oxidation and reduction half reactions, we can use an equation
to determine the overall cell potential for a galvanic cell consisting of magnesium
and silver. 𝐸 cell is equal to 𝐸 cathode minus 𝐸 anode. We can now put in our values. 𝐸 cathode is the potential of silver, which is positive 0.7996 volts. And 𝐸 anode is the potential of magnesium, which is negative 2.372 volts.
Be careful, we have two minus signs. Solving, we get positive 3.172 volts, which is the standard cell potential for a
galvanic cell consisting of magnesium and silver. This is the maximum potential difference between the magnesium and silver electrodes
at the beginning of the redox reaction. Over time, as the reaction proceeds, this value will decrease. The positive sign in the cell potential tells us that when magnesium is the anode and
silver the cathode, a spontaneous reaction occurs.
Finally, what is the standard cell potential? The answer is positive 3.172 volts.
